Deserts breed strange visions. Mystics have seen gods, angels, and devils in whirlwinds of sand and flickers of heat lightning. Soldiers have seen great fortresses, carved by giants from civilizations past, in weathered sandstone cliffs. Conquerors have seen cities of gold beckoning from the edges of salt pans and the summits of sand dunes.

And desert rats from Lawrence of Arabia to Bugs Bunny have seen white castles, gleaming cities, and even belly dancers floating on air above the scorching ground—and, more beguilingly still, pools of water where only sand and rock await.

These visions speak to a fact of the desert: such places are disorienting, and for reasons both meteorological and geological. If you stand at a high point in a field in, say, western New Jersey or the hill country of southern Germany, places where the sky is full of moisture and the relief is broken by low rises and trees, your vision will be limited to something under three miles in any given direction. From a similarly high point in the middle of a valley in the basin-and-range deserts of the American Southwest or in the central Sahara, however, where rocky plains are bounded by mountains fifteen or twenty miles distant, that fifty-odd-square-mile visual domain can grow a thousandfold. The absence of water in the atmosphere brings what would elsewhere be a hazy horizon into sharp focus, so that those faraway mountains stand out in sharp relief against the sky. It’s hard to judge distances in such conditions, and many a traveler has perished by underestimating the amount of hot, waterless ground to be covered.

That vastness alone is enough to confuse many travelers. But add to it another meteorological curiosity of the desert: the air cools rapidly just inches above the hot surface, and just a foot off the ground it is a full 30° F cooler than the desert floor. This quick cooling yields layers of air masses of differing temperatures and, consequently, density. The ensuing turbulence as relatively hotter and cooler layers collide causes light to bend, lending the air a visible, slightly hallucinatory tremble. (On a summer day just about anywhere warm, desert or not, you can see the same effect above a sun-heated asphalt road.) If the air masses are sufficiently different in temperature, the light can bend so far that it turns back on itself, reflecting the sky above or nearby objects. That mirror-image phenomenon is called a mirage, after the French word meaning “reflection.”

Mirages, strictly defined as refraction phenomena in which two or more images of the same object appear, form early in the day in the desert heat. The most abundant species of mirage lends credence to that strict definition: caused by inverted light above an erect object—a telephone pole, say, or a tree or cactus—it gives the impression that a half-mile or so ahead stands a thicket of such things. Another common species, caused by a more subtle distortion of light, gives a quite different impression: that a mountain range many miles distant has somehow moved much closer to the oncoming traveler, though, strangely, it keeps on receding as that traveler advances.

Strictly speaking, mirages are not optical illusions; they can be photographed, though it takes some skill with a camera to do so. Neither are they confined to dry or even hot places; the most important driving mechanism for them is simply the presence of differentially heated air masses. At sea, for instance, where the presence of water complicates the shifting air masses, a mix or distorted and inverted images can yield the “Fata Morgana” phenomenon, which can lead to near-miraculous sights of cities in the clouds—or, disconcertingly, of looming icebergs.

The Fata Morgana (poetically named after the Welsh wizard Merlin‘s sorceress half-sister, who, like the sea, had a murderous streak) is common at all latitudes. Still, observed the Dutch geographer B. G. Escher—the brother, fittingly enough, of the famed optical-illusion artist M. C. Escher—such mirages seem to be “greatest in the regions where the precipitation is least, that is, where the shipping routes pass near or between deserts. . . . A dry climate is favorable for the appearance of this very remarkable phenomenon.”

And so we’re back to the desert after all, where bent and broken light offers a particularly cruel vision: that of imaginary lakes, stretching out invitingly and endlessly. This familiar phenomenon is caused by light mirroring, at an oblique angle, a particularly clear sky. Stand and study it closely, and you’ll see that the surface of the desert appears to be pitch-black below the line of distortion. Above it, the shimmering air “floats,” looking maddeningly like water—an impression reinforced by the presence of “islands,” which are really the distorted images of the rocks and pebbles that lie scattered on the desert floor.

The lost-lake phenomenon, interestingly, often makes itself visible where ancient lakes stood in the distant days when the deserts were wetter places. The ancient lakebeds, called playas after the Spanish word for beach, are perfect venues for mirage creation, lying low and flat, gathering comparatively cold air (which sinks to the lowest available point) at night and providing a frying pan in which that air can be rapidly heated by day. One such place is the Black Rock Desert of northwestern Nevada, where, in 1849, a California-bound traveler reported seeing a curious vision: a “long lagoon of blue water.” The lagoon was not there, of course, but so realistic was the mirage that his oxen stampeded toward it, “hoping to quench their burning thirst, and left their swelled up carcasses over the plain.”

Without those oxen, the traveler nearly met their fate, but he lived to tell the tale. And a mysterious tale mirages make, one that even the driest scientific explanation does nothing to diminish.